Displacement sensors, such as microphones and pressure sensors, are well-known. Displacement sensors based on one or more of electrical capacitance, electrical impedance, or magnetic fields have been in use for many years. More recently, optical displacement sensors have been developed. Optical displacement sensors are particularly attractive as they overcome many of the limitations associated with other measurement techniques; limitations such as low sensitivity, the need for high-voltage biasing, poor electrical isolation, or response nonlinearities. Optical interferometers have been proven to have outstanding resolution when used as displacement detectors in physical sensing components, such as microphones, pressure sensors, and accelerometers.
Many optical-displacement sensors known in the prior art operate by detecting light reflected by an optical element that changes its reflectivity in response to an environmental stimulus, such as pressure differential, sound, vibration, etc. The detected light is converted into an electrical signal. This signal is a function of the reflectivity of the optical element, and, therefore, a function of the stimulus as well.
A Fabry-Perot interferometer has served as such an optical element. A Fabry-Perot interferometer is an optically resonant cavity that distributes optical energy of an input light signal into a reflected signal and a transmitted signal. The ratio of optical energy in the reflected and transmitted signals depends on the cavity length of the optically resonant cavity, which is the spacing between its two, substantially-parallel, partially reflective surfaces and its operating wavelength, λ, (i.e., the wavelength, λ, of the light on which the interferometer operates).
In order to form a Fabry-Perot interferometer that is sensitive to sound, etc., one surface of the Fabry-Perot interferometer is a surface of, or disposed on, a movable element. When the element moves in response to the environmental stimulus, the cavity length changes and, therefore, so does the ratio of optical energy in the reflected and transmitted signals. As a result, an electrical output signal based on one of the reflected and transmitted signals is a function of the environmental stimulus incident on the Fabry-Perot interferometer.
Unfortunately, interferometer-based sensors have some known drawbacks that limit their sensitivity. Source noise associated with a light source that provides the input signal to the interferometer can be very difficult to distinguish from a motion of the movable element. This reduces the signal-to-noise ratio (SNR) of a sensor. Similarly, detector noise associated with the photodetector that generates the electrical output signal can also reduce the SNR of the sensor. Further, the linear range of operation of a Fabry-Perot interferometer-based sensor is typically much less than the wavelength of light in the input signal.
A Fabry-Perot interferometer-based displacement sensor having high dynamic range and high sensitivity would be a significant advance in the art.